N-Aryloxyamines have been prepared in the prior art by the reaction of a phenylhydrazine with a stable nitroxide. In certain instances, N-aryloxyamines have been prepared in low yield by the decomposition of aryldiazonium salts in the presence of a nitroxyl radical, but without a transition metal catalyst present. The focus of these papers is on mechanistic studies. The products are obtained in poor yields and the reaction is limited in scope. A. C. Scott et al., J. Chem. Soc., Perkin Trans. 2, 1980, 260-266.
Beckwith et al. reported the intramolecular addition of an aryl radical formed by decomposition of a diazonium salt to a double bond. The resultant alkyl radical is trapped by a stable nitroxyl radical. L. J. Beckwith and G. F. Meijs, J. Chem. Soc., Chem. Commun. 1981, 595-597.
In co-pending application Ser. No. 09/824,149, a transition metal-catalyzed process is disclosed for a commercially viable preparation of N-substituted aryloxyamines by the reaction of an aryldiazonium ion with a sterically hindered nitroxyl radical. The transition metals include copper(I), copper(II), cobalt(II), manganese(II), titanium(III), iron(II), iron(III), cobalt(III), nickel(II), gold(I) or chromium(III).
Surprisingly, the yield of aryloxyamine using the catalysts of the instant invention, for the reaction of aryldiazonium ions with sterically hindered nitroxyl radicals is equivalent to or greater than that for simple transition metal salts. This is especially surprising considering that concentration of the “transition metal” is significantly lower in the instant invention. For example application Ser. No. 09/824,149 demonstrates that the use of ferrous sulfate as a catalyst in the process to produce sterically hindered aryloxyamines results in a less than 50% yield. In the instant invention, the Fe(II) substituted polyoxometalate, K7PFe(II)W11O39, results in a 72.7% isolated yield of the aryloxyamine. The percentage of transition metal of the polyoxometalate catalysts of the instant invention is much lower than the percentage of transition metal in the simple transition metals salts used in co-pending application Ser. No. 09/824,149, i.e. the Fe(II) substituted polyoxometalate contains only 1.85% iron compared to 20% iron in ferrous sulfate heptahydrate. Despite the lower concentration of transition metals in the polyoxometalate catalysts of the instant invention compared to the concentrations found in co-pending application Ser. No. 09/824,149, high yields are still obtained.
Polyoxometalates (POMs) are large metal oxide clusters of the early transition metals; tungsten(VI), molybdenum(VI), vanadium(V), niobium(V) or tantalum(V). POMs are formed by linking MO6 octahedra together via either corner shared, edge shared or face shared linkages. The structure and catalytic properties of transition metal substituted polyoxometalates has been extensively reviewed (Neumann, Ronny, Prog. Inorg. Chem. (1998), 47 317-370.; Sadakane, Masahiro; Steckhan, Eberhard. Chem. Rev. (Washington, D.C.) (1998), 98(1), 219-237; L. I. Kuznetsova et al. J. Mol. Cat. A: Chem. 1997, 117, 389; C. L. Hill and M. Prosser-McCartha, Coordination Chemistry Reviews 1995 143, 407-455 and Hill, Craig L.; Kim, Gyu-Shik; Prosser-McCartha, Christina M.; Judd, Debbie. Mol. Eng. (1993), 3(1-3), 263-75). Other reports discussing the structure, synthesis, and applications of polyoxometalates include (a) Cramarossa, M. R et. al. J. Mol. Catal. A: Chem. (1997), 127(1-3), 85-94, which describes the homogeneous oxidation of cyclohexane and adamantane transition metal-modified, Keggin-type heteropoly complexes; (b) Lyons et al., in U.S. Pat. No. 4,803,187, which describes the use of POMs for the oxidation of alkanes to alcohols or ketones; (c) Duncan, Dean C.; Chambers, R. Carlisle; Hecht, Eric; Hill, Craig L. J. Am. Chem. Soc. (1995), 117(2), 681-91, which reports the mechanism of olefin epoxidation by hydrogen peroxide and H3[PW12O40]; (d) X. Zhang et al. Inorg. Chem. 2001, 40, 418, which describes the use of [(n-C4H9)4N]6[FeIII4(H2O)2(PW9O34)2] for the oxidation of alkenes using hydrogen peroxide; and (e) D. A. Judd, et al. J. Am. Chem. Soc. 1997, 119, 5461, which reports the synthesis of [P2W12(NbO2)6O56]12−. These references in general describe oxidative applications of polyoxometalates unrelated to the instant invention. The use of polyoxometalates for the deoxygenation of aldehydes and ketones is described by V. Kogan Angew. Chem. Int. Ed. 1999, 38, 3331 and is unrelated to the instant invention.
It is clear that the instant invention provides an improved class of catalysts for the preparation of sterically hindered aryloxyamines described in co-pending application Ser. No. 09/824,149. The disclosure of U.S. application Ser. No. 09/824,149 is hereby incorporated by reference.